Hi. I'd like to know what is the safest assumption that I can make in order for me to make sure that my transistors are saturating. Please help me so that I can continue this design that I'm doing for a common-emitter astable multivibrator switch. I'm using 2N3904 NPN transistor which has 0.2 V as the saturation voltage for the CE junction (with Ic(sat) = 10 mAdc). Is it safe to compute for the collector resistance by assuming that the voltage across the CE junction is 0.1 V? If so, once I get Rc, how do I get Rb? By the way, my circuit has an emitter resistance, Re, which is present to compensate for the citcuit's symmetric functionality (so that the two transistors won't turn on simultaneously). Thanks a lot.

 

The value of the base resistor determines how well the transistor saturates. The 2N3904 transistor and most others are spec'd to saturate well with the base current at 1/10th the collector current. So for 10mA collector current, the base current is 1mA. Calculate the voltage across the base resistor then use Ohm's Law to calculate its value.

Caution: The max reverse emitter-base voltage for the 2N3904 and most other silicon transistors is only 6V. After subtracting the voltage across the emitter resistor, if 6V or more will occur then protective diodes are recommended.

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Uncle $crooge